The present invention relates to an acidic equilibration catalyst method for equilibrating organopolysiloxane having aromatic organic radicals attached to silicon by carbon-silicon bonds with poly(organosiloxane) having aliphatic organic radicals attached to silicon by carbon-silicone bonds. More particularly, the present invention relates to a method of reducing cleavage of aromatic organic radicals attached to silicon by carbon-silicon bonds during equilibration, by delaying the addition of such aromatic organic siloxane to the acidic equilibration mixture.
Prior to the present invention, a satisfactory procedure was not available for acid equilibrating organopolysiloxane having aromatic organic radicals attached to silicon without "protodesilation" occurring. In instances where a basic equilibration catalyst such as potassium hydroxide is used, the problem of protodesilation or acid cleavage is avoided. However, basic equilibration cannot be used effectively in instances where the organopolysiloxane has base sensitive functional groups, such as aromatic anhydrides, attached to silicon.
It would be desirable, therefore, to provide a satisfactory procedure for acid equilibrating aromatic organic-substituted siloxanes with poly(aliphatic organic-siloxane)s which minimizes protodesilation.
The present invention is based on the discovery that acid equilibration of poly(aliphatic organicsiloxane) and organicsiloxane having aromatic organic radicals attached to silicon by carbon-silicon bonds, such as phthalic anhydride-substituted organicsiloxane, can be achieved without a significant degree of acid cleavage of phthalic anhydride groups occurring. It has been found that protodesilation of aromatic organic groups from organicsiloxane can be reduced by delaying the addition of the aromatic organic radical-substituted organicsiloxane to the equilibration mixture. If the equilibration catalyst, such as trifluoromethanesulfonic acid, is initially dispersed in the poly(organicsiloxane) substantially free of aromatic organic substitution and the resulting mixture is then blended with poly(aromatic organic siloxane), a significant reduction in acid cleavage of aromatic organic radicals is achieved. It also has been found that alkenyl radicals such as vinyl attached to silicon by carbon-silicon bonds also are less likely to cleave under acidic equilibration conditions if the addition of the alkenyl siloxane is delayed in a similar manner.